On highway trucks often have a power-take-off ("PTO") or other devices which allow the truck engine to power devices connected to the truck. The PTO generally includes a rotating shaft extending from the transmission or engine. The device to be powered is then connected to the PTO shaft. As the PTO shaft rotates, power is transmitted to the device. One such device is a hydraulic pump used in connection with a garbage truck. In that application, the PTO shaft is connected to the hydraulic pump which in turn provides hydraulic pressure to the hydraulic cylinders that compact the garbage. Other vehicles that have a PTO include utility vehicles, school buses and fire trucks.
In these applications, the truck engine provides power to both the PTO and the drive train of the truck. In cases where the truck is traveling on the road, the engine speed is generally high enough to provide sufficient power for both the PTO and the transmission. However, when the truck is parked and idling, the engine may not produce sufficient power to drive the external device connected to the PTO. In those instances, the operator has sometimes been required to press the accelerator pedal to elevate the idle speed to cause the engine to produce more power. When using the throttle, however, it is possible to generate too much power which could damage the external device. Manually maintaining a desired and acceptable elevated idle speed is inefficient and prevents the operator from performing other tasks when the truck is stopped and idling.
There are known prior art devices that have to some extent addressed this problem. One such device is disclosed in U.S. Pat. No. 5,480,364, in which an apparatus for selectively commanding an elevated high idle mode is disclosed. That apparatus preferably includes a toggle switch or other means for causing the engine to enter a high idle mode. High idle mode is normally selectable when the brake pedal is not pressed, the clutch pedal is not pressed, the vehicle is traveling below a certain predetermined speed, and the gear selector is in neutral. In a preferred embodiment of that invention, if the engine speed is below a default value when the toggle switch is activated, then the idle speed will correspond to the default valve. If the engine speed exceeds a maximum high idle value when the toggle switch is activated, then the idle speed will correspond to the maximum high idle value. If the engine speed is greater than the default value but less than the maximum high idle value when the toggle switch is activated, then the idle speed will correspond to the speed of the engine at the time when the toggle switch is activated.
Other known PTO control systems that are implemented on an electronically controlled engine permit the end user or other qualified personnel to program a number of predetermined desired PTO engine speeds. For example, the end use may involve a garbage truck that requires the engine to run at several different speeds to most efficiently load and compact garbage. In that case the end user may program the engine to run at the predetermined speeds of idle, 1200 and 1600 RPM. Then when the vehicle operator causes the controller to run in a PTO mode and toggles the PTO switch the engine speed command will generally increase to the next higher engine speed, except when the desired engine speed is 1600 RPM, in which case toggling the switch will cause the engine to return to the lowest preset speed. Thus, if the engine is idling at its normal idle speed and the PTO device requires more power, the operator can simply toggle the PTO switch to cause the engine speed to increase to the next programmed step, in this case 1200 RPM. If the operator then toggles the PTO switch again, then the engine speed will increase to the next programmed step, in this example 1600 RPM. Another toggling of the PTO switch will cause engine speed to return to idle. In this way, the operator can use the PTO switch to toggle between the preprogrammed PTO engine speeds.
The above system overcomes some disadvantages associated with the prior art, but still requires manual intervention by the vehicle operator to modify the engine speed. It would be preferable to have an apparatus that could cause the engine to automatically vary the engine speed based on the operational mode of, or power required by, the PTO device.